refactor(admin): drop AI connection config page for WP AI Connectors

Remove the "AI 连接配置" page under 扩展&增强 together with the theme's
own OpenAI-compatible provider path. AI generation (SEO, summary, tags)
now runs solely on the WordPress AI Client (wp_ai_client_prompt), with
connections configured through WordPress AI Connectors.

- AI.php keeps only the WP AI Client path: no provider config storage,
  no theme fallback, no rate limiting
- Drop /ai/status, /ai/settings and legacy /ai/seo-suggest routes plus
  their handlers
- Remove dead compat code: AISEO::suggest, AIPrompt::build_seo_prompt
  and the front-submit AI assist flow (its buttons were never rendered
  and its endpoints were never registered)
- Update SEO schema/meta-box guidance that pointed at the deleted page
- Add ai-config-removal contract test and refresh the toast and
  user-features contracts affected by the cleanup

templates/parts/submit/context.php

@@ -177,8 +177,6 @@ if ($is_guest) {
 	$submit_label = __('保存', 'aksrt');
 }
 
-$ai_available = class_exists('\AKSRT\Ai\AI') && \AKSRT\Ai\AI::get_instance()->is_available();
-
 return [
 	'front'              => $front,
 	'user_id'            => $user_id,
@@ -207,7 +205,6 @@ return [
 	'vip'                => $vip,
 	'points_enabled'     => $points_enabled,
 	'submit_label'       => $submit_label,
-	'ai_available'       => $ai_available,
 	'can_pay'            => $cap('new_post_pay'),
 	'can_pay_download'   => $cap('new_post_pay_download'),
 	'can_cover_image'    => $cap('new_post_image_cover'),